1. Field of the Invention
The invention relates generally to high air-fraction non-silica-based glass fibers.
2. Description of the Prior Art
Hollow core photonic band gap (HC-PBG) fibers have been fabricated from silica glass and reported in the literature (Cregan et al., “Single-mode photonic band gap guidance of light in air,” Science, 285(5433), 1537-1539 (1999); Barkou et al., “Silica-air photonic crystal fiber design that permits waveguiding by a true photonic bandgap effect,” Optics Letters, 24(1), 46-48 (1999); Venkataraman et al., “Low loss (13 dB/km) air core photonic band-gap fibre,” ECOC, Postdeadline Paper PD 1. 1, Sep., 2002. All referenced publications and patents are incorporated herein by reference). FIG. 1 shows a schematic of the cross-section of a HC-PBG fiber. The periodic layered structure of holes and glass creates a photonic band gap that prevents light from propagating in the structured region (analogous to a 2D grating) and so light is confined to the hollow core. Typically, the periodicity of the holes is on the scale of the wavelength of light and the outer glass is used for providing mechanical integrity to the fiber. The fact that light travels in the hollow core also means that the losses will be lower so longer path lengths can be used. Also, non-linear effects will be negligible and damage thresholds will be higher so that higher power laser energy can be transmitted through the fiber for military and commercial applications. Also, since light is guided in the hollow core, an analyte disposed therein will have maximum interaction with light, unlike traditional evanescent sensors.
The periodicity of the holes, the air fill fraction and the refractive index of the glass dictate the position of the photonic band gap or gaps, namely the transmission wavelengths guided through the hollow core. PBG fibers are obtained by first making a microstructured preform and then drawing this into fiber with the correct overall dimensions. In some cases, the air fraction needed in the fiber, and therefore preform, is as high as 90% or even higher to provide a photonic band gap. An example includes chalcogenide glass PBG fiber for transmission in the infrared region beyond 2 μm. Irrespective of the technique used to make the preform, it is very difficult to make high air fraction preforms, especially from specialty glasses such as chalcogenides, halides, chalcohalides, and the like. Unlike silica, which is a relatively strong material, specialty oxide and non-oxide glasses may be weaker and become difficult to fabricate, and moreover, difficult to handle when the air fraction is so high. Consequently, there needs to be a technique suitable for making high air fraction fiber, from specialty glasses.